1. Field of the Invention
This application is directed to lubricant compositions and to a method of improving the stability of synthetic lube base stocks. This application is more particularly directed to alkylated aromatic base fluids as blending stocks with polyalphaolefin base fluids thereby providing synthetic lubricant compositions having significantly improved oxidation stability, solubility, elastomer compatibility and hydrolytic stability.
2. Description of Related Art
Synthetic hydrocarbon fluids useful as lubricant compositions are well know in the art. For example U.S. Pat. No. 3,149,178 (Hamilton et al.) discloses that thermally or catalytically polymerized alpha monoolefins provide lubricants having low pour points and high viscosity indices which nevertheless are not sufficiently stable to high temperature lubrication conditions and in some cases are insufficiently responsive to additives. Its solution to these problems is to remove the dimer portion of polymerized alpha monoolefins prior to hydrogenation and heat treat the product.
Further, various blends of one or more polyalphaolefins and esters plus additive packages have long been commercially available. Polyalphaolefin (PAO-based) lube products are often blended with carboxylic acid esters to improve the solvency of PAO base stocks, but, the addition of the esters causes reduced thermal/oxidation stability and hydrolytic stability of the PAO/ester blends. Also, alkylaromatic fluids have been proposed for use as certain types of functional fluids where good thermal and oxidative characteristics are required; see, for example, U.S. Pat. No. 4,714,794 (Yoshida et al.). The use of a mixture of monoalkylated and polyalkylated naphthalene as a base for synthetic functional fluids is also described in U.S. Pat. No. 4,604,491 (Dressler).
This invention provides PAO-based lube products of improved thermal/oxidation stability and hydrolytic stability comprising blends of PAO and alkylated aromatic base stocks.
To our knowledge, this thermal/oxidation stability improvement is unexpected and has not been demonstrated heretofore.